board: amlogic: add support for Odroid-N2

ODROID-N2 is a single board computer manufactured by Hardkernel Co. Ltd
with the following specifications:

 - Amlogic S922X ARM Cortex-A53 dual-core + Cortex-A73 quad-core SoC
 - 4GB DDR4 SDRAM
 - Gigabit Ethernet
 - HDMI 2.1 4K/60Hz display
 - 40-pin GPIO header
 - 4 x USB 3.0 Host, 1 x USB OTG
 - eMMC, microSD
 - Infrared receiver

The board directory is W400, the name of the Amlogic Reference Design
of Amlogic G12B with Gigabit boards, which will be used for similar
boards.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Mark Kettenis <kettenis@openbsd.org>